Tornado usually occurs in unstable, high temperature and humid air mass, the great temperature difference between ground and high altitude causes the rapid drop of cold air and the speedy rise of hot air, then strong air convection generates a strong wind vortex with a high-speed rotating funnel-shape cloud column.
Tornado is generation of thunderstorm in cloud, to be specific, tornado is a form of energy that released intensively in a small area from the part of tremendous energy of thunderstorm. The generation of tornado could be divided into 4 steps:
(1). The instability of atmosphere generates strong upwind, which is strengthened strengthen by the effects of peak transit flow in supercritical flow.
(2). The updraft begins to rotate in the middle of troposphere due to interaction with the wind which has shear transformation on velocity and direction in vertical direction, and forms mesocyclone.
(3). With the extension to ground and sky, the mesocyclone turns to be thinner and stronger. Meanwhile, an air enhancement in a small area will generate a cyclone, and then turn into the core of tornado.
(4). The rotation of tornado core is different from that of cyclone, it is strong enough to extend tornado to the ground. When the developing vortex reaches ground, the pressure on ground decrease rapidly, and wind speed rise sharply, finally a tornado with strong cyclone is formed.
To analyze from electric field—one formation condition of tornado: only huge cumulonimbus could generate tornado, in the process of discharging between cloud and sky, the positive charge on top of cloud is far more than negative charge under cloud, after charge neutralization of lightning, there is lack of negative charge under cloud, and the cloud with a large number of positive charge will generate strong electric field between cloud and earth. Under the effect of electrostatic attraction, the cloud with positive charge will extend to earth, then the atmosphere with negative charge will be gathered for charge neutralization. Finally, there will be a funnel cloud, with high speed rotating atmosphere, generated from bottom of cumulonimbus.
The cumulonimbus will extend to earth or sea rapidly if there is enough positive charge in cloud, then the tornado will come when it reaches ground or water surface. The cloud column tornado is cloud cluster airflow moving downwards with plenty of positive charge, there is a strong electric field between cloud and earth. Although the electric field is not strong enough to generate lightning, it could cause powerful negative ions flow on earth or sea, after generation of negative ions flow, the air rises sharply and produces an area of low pressure, and ambient air would be gathered to this area under the action of atmospheric pressure, gathered air rise rapidly under negative ions flow, and gathered airflow is affected by deflecting force of earth rotation. Tornado counterclockwise rotates in the northern hemisphere, and clockwise rotates in the southern hemisphere. In this way, the pressure in the bottom of tornado gets lower and lower, and wind speed gets faster and faster.
Tornado is one of the strongest vortex phenomenon in atmosphere, and has strong effect of sucking. The ambient air rotates around axis of tornado quickly, under the attraction of pressure decease from tornado core, air would be sucked into vortex bottom near the earth in a dozens of meters thick layer, then become the upward vortex around the tornado axis. The pressure in tornado center is ten percent lower than that of ambient air.
United States is a tornado-prone area. Geology Professor Randy from Arizona State University explained that there are 80% to 90% tornado occurred in America from world wide range. There are 5 tornado on average occurs every day, and for years there will be 1000 to 2000 tornado, especially in Oklahoma, Kansas, Nebraska and Iowa. American tornado is not only known as quantity, but also strength, so this make America be ‘hometown of tornado.
Tornado is a kind of catastrophic climate, which contain a large amount of energy. When the diameter of funnel vortex reach 200 meters, its cyclone power could be 30000 MW, which is equal to 10 large hydropower station's total electricity capacity. At the moment, the main method to defense tornado disaster are focusing on monitor and prediction, however, there is lack of man-made interference to reduce the power of tornado.